Magnetic Resonance Imaging (“MRI”) has emerged as a leading medical imaging technology for the detection and assessment of many pathological and physiological alterations in living tissue, including many types of tumors, injuries, brain-related conditions, coronary conditions, and orthopedic conditions, among others. According to current medical knowledge, an MRI scan of a patient is non-invasive and harmless.
As know to those skilled in the art, the MRI scan generally utilizes magnetic and radio frequency (“RF”) fields to elicit a response from a given patient's tissue, and to provide high quality image “slices,” i.e., two-dimensional image reconstructions of a two-dimensional cross-section of the patient's body, e.g., a tissue along with detailed metabolic and anatomical information. The slices are formed from the transmission of radio waves, in combination with a magnetic field that is about 10,000-30,000 times stronger than the magnetic field of the earth, through the patient's body. This affects the patient's atoms by forcing the spins of the nuclei of some of the atoms into a different position. When such nuclear spins move back into place, they transmit their own radio waves. An MRI scanner receives those radio waves, and a computer associated with the MRI scanner transforms them into images, based on the location and strength of the incoming radio waves. A three-dimensional slab can be also be encoded by a combination of magnetic field gradients and RF pulses.
MRI scans offer high spatial resolution, superior anatomical detail of soft tissues as compared to other medical imaging technologies, and are able to acquire images in any plane. These scans, however, may be significantly affected by motion artifacts, such as the patient's respiration, cardiac cycle and physical movement. Such motion artifacts may cause problems in many MRI applications, including FMRI, cardiac and abdominal MRIs, and long repetition time (“TR”) acquisitions, among others.
Motion artifacts may be reduced or compensated for with the use of several techniques, such as physiologic gating, phase-encode recording, fiducial markers, fast acquisitions, image volume registration, or other alternatives, including navigator-based techniques. The navigator-based techniques generally use k-space or image space navigators for detecting motion during image data acquisition. A navigator is a rapidly-acquired sequence of the anatomical object being scanned, e.g., the patient's head, representing a projection of the image data in k-space or image space and from which the position of the object may be deduced along with other information such as B0 drift, shim offsets and information related to physiological activity. A navigator signal may be produced in each pulse sequence along with the image signal. A data set may be acquired for both. The physiological motion causes global displacement in the navigator signal, and results in a shift of the navigator signal.
The data captured by the navigator can be used to detect rotational and translational motion in the plane and to correct for motion artifacts, either retrospectively or prospectively. A motion correction during the acquisition of a single volume should be performed prospectively as parts of k-space are omitted if motion occurs during an uncorrected scan.
The earliest navigator-based techniques for correcting motion artifacts have utilized straight-line navigator echoes to detect a linear motion. Such linear techniques may be useful in chest examinations where the diaphragm and associated organs translate along a particular axis. However, these conventional linear techniques do not quantify or determine the magnitudes or degrees of rotations of the objects being scanned, or portions thereof.
For example, U.S. Pat. No. 4,937,526 describes a similar conventional technique that uses a method for reducing motion artifacts in MRI images, in which the MRI data set used to reconstruct the image is corrected after its acquisition using information acquired concurrently by a navigator signal. This navigator signal described in the patent is a projection along an axis defined by the readout gradient which is fixed in direction throughout the scan. As a result, the navigator signal is only able to detect motion linearly, along the direction of the readout gradient, and is not able to provide rotational motion information, which is critical when performing certain MRI scans such as those of a patient's heart or brain.
The difficulty in correcting for a rotational motion has been ameliorated with the use of circular or orbital navigators. In one example, as described in U.S. Pat. No. 5,539,312, navigator signals are acquired using a unique pulse sequence which samples two-dimensional k-space in a circular trajectory. These circular navigator signals are used to correct image data for rotation and translation in a single two-dimensional plane. To obtain sufficient information to correct for all possible rotations and translations, three of these circular navigators are required to characterize the object motion about three cardinal axes. While this approach fulfills the theoretical need to compensate for all three axes of motions, it is relatively impractical because the entire procedure is time-consuming.
A preferred approach may be to use more sophisticated navigators to capture the translations and rotations of the object fully. For example, U.S. Pat. No. 6,771,068, the entire disclosure of which is incorporated herein by reference, describes that the navigator can be an octant navigator that traces the outline of an octant on the surface of a sphere in k-space. The octant navigator enables a rotation about the three cardinal axes and a translation in all three directions to be achieved in a single read after a single radio frequency pulse. A pre-mapping of the k-space in a small number of degrees in each direction from the initial octant navigator is generated to eliminate the need for an iterative, approximate solution. By comparing the actual navigator with a local pre-mapped k-space map, it is possible to determine the true rotations and translations using a single subsequent octant navigator. The octant navigator can be applicable in two- and three-dimensional sequences for motion correction.
In addition, U.S. patent application Ser. No. 10/846,372, the entire disclosure of which is incorporated herein by reference, describes a “clover leaf” navigator. The clover leaf navigator traces a path through k-space (or a phase space) that includes a straight-line section in each direction through the center of k-space to gauge translations, and may also include approximately ninety-degree arcs in three perpendicular planes to gauge rotations. These rotations can be described using quaternions to avoid the problem of “gimbal lock,” which can occur when angle rotations that are described relative to the cardinal axes result in an alignment of two axes such that a degree of freedom is lost. The object's motion is then calculated using a rapid and robust linear method.
While the clover leaf navigator may provide better translation and rotational motion estimates than the octant navigator, such estimates do not remove out-of-plane effects from the navigator. The estimates may also be inaccurate if shifts in position of the object in the B0 field after shimming occur. Those shifts may invalidate the shim and result in offsets in the navigator trajectory in k-space and artifacts in the image. The navigator may also be affected by phase encoding gradients that change with every line of the image, and drift in the B0 field that occurs with heating of the shim iron during a high resolution scanning with large gradients or as a consequence of physiology such as breathing.
Thus, there is a need to provide a system, method, and computer-accessible medium for correcting motion artifacts during MRI scans in real-time by using the navigators that accurately estimate translations and rotations of the object being scanned. There is a further need to correct the motion artifacts during MRI scans in real-time by using the navigators that account for out-of-plane and phase encoding effects, shimming errors and B0 drifts. There is yet a further need to correct the motion artifacts during MRI scans in real-time by using the navigators when multiple coils are present.